1. Field of the Invention
This invention relates to a method of treating the surface of an iron alloy material, as for making dies, jigs, tools or machine parts, to form thereon a layer composed of a nitride or carbonitride of one or more of the surface layer-forming elements, i.e., chromium, Group Va metals (vanadium, niobium and tantalum), titanium and zirconium.
2. Description of the Prior Art
It is well known that if the surface of an iron alloy material is covered with a layer composed of a carbide, nitride or carbonitride of one or more of the surface layer-forming elements, it improves the wear, seizure, oxidation and corrosion resistances and other properties of the iron alloy material. There have been proposed many methods for forming such a surface layer.
For example, a layer of chromium carbide is formed on a iron alloy material by immersion in a salt bath composed of a chloride (see Japanese Laid-Open Patent Specification No. 200555/1982 or 197264/1983). An iron alloy material which has been nitrided is chromized, whereby a layer of chromium carbonitride is formed on its surface (see Japanese Patent Publication No. 24967/1967 or U.S. Pat. No. 4,242,151).
Both of these methods, however, involve heating at a temperature which is higher than the Ac.sub.1 transformation point of iron (about 700.degree. C.). The heat develops strain in the iron alloy material and if it has a complicated shape, it is very likely to crack. The use of such a high temperature presents other problems, too, including the worsening of the working environment.
There have, therefore, been proposed a number of methods for forming a surface layer at a temperature not exceeding 700.degree. C. They employ, for example, a halide of the surface layer-forming element or elements and form a surface layer by, for example, CVD (chemical vapor deposition), plasma CVD, ion plating or PVD (physical vapor deposition) [see, for example, Japanese Laid-Open Patent Specification No. 65357/1980, 154563/1980 or 151469/1983, "Kinzoku Hyomen Gijutsu" (Metal Surface Technology), No. 2, p.28 (1979), or Japanese Laid-Open Patent Specification No. 2715/1980 or 164072/1980].
Although all of these methods can form a surface layer on an iron alloy material without producing any thermal strain, they can hardly form a surface layer which is satisfactory from the standpoints of thickness uniformity and adhesive strength. All of them are complicated and require expensive equipment. Moreover, they are inefficient, as they need be carried out in the presence of hydrogen or at a reduced pressure.